NNTP gatewaying trashes Message-IDs

when a message is relayed to NNTP, the NewsRunner
unconditionally replaces the existing Message-ID with
its own. this breaking threading when reading a list
via news, and when an NNTP user replies to a list message.

a better solution would be either:
1) do not rewrite the message-id unless the NNTP server
reports an error when posting it (duplicate ID); or
2) optionally, always rewrite posters' message-ids into
a mailman-generated ID, and use this ID for both mail
and news.

i have implemented the former at our site and it
appears to work, but it's so ugly i'd not feel
comfortable submitting a patch for it.